$blogs = wp_get_sites();
$postType = array('event', 'ohmama');
$globalPosts = array();
foreach($blogs as $blog) {
    switch_to_blog($blog['blog_id']);
    $args = array(
        'post_type'     => array('event', 'post'),
        'category'           => get_cat_ID('Global')
    );
    $results = get_posts($args);
    foreach($results as $result1) {
        $result1->blog_id = $blog['blog_id'];
        $globalPosts[] = $result1;
    }
    restore_current_blog();
}
function orderDate ($a, $b) {
    $aTime = strtotime($a->post_date);
    $bTime = strtotime($b->post_date);
    if ($aTime == $bTime) {
        return 0;
    }
    return ($aTime > $bTime) ? -1 : 1;
}

usort($globalPosts, 'orderDate');